By assembling diverse factors that influence the group composition of cercopithecines, the socioecology of cercopithecines can be understood through the investigation of the relationship between group composition and members’ fitness output. In a favorable socioecological condition, if the number of receptive females that are capable of being monopolized by a single male increase, males would be favored to live in a group where it contains a smaller number of males and is less hierarchically stratified. On contrary, when the monopolization of mating opportunities is threatened by extra group copulation, the influx of adult male immigrants, and overthrown of alpha position, cooperation in antagonistic interaction among the senior resident males may be favored. The equations, hypotheses, and predictions in the present study provide a theoretical framework that suggests a double-facet function of dominance hierarchy: the queue and the solidarity. And the hypothesized two facets are supported by a strong positive correlation between age and dominance rank from a meta-analysis on studies of 11 macaque troops and an independent analysis on 5 baboons troops, and are detailed by the identification of the stratification of dominance rank in relation to the age and seniority of males. Taken together, the highest dominance rank tends to be achieved by macaques and baboons at their prime age (approximately 10 y), and males with a longer cohabitation (seniority) tend to occupy adjacent dominance ranks. A use of naturalistic observation and controlled experimentation in the future may specify whether the stratification of a dominance hierarchy in relation to seniority is likely resulted from male-male collation in antagonistic interaction, as predicted by the solidarity facet, and may deepen our understanding of the predictive power of dominance rank on males’ mating opportunity and reproductive output.




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